The present invention pertains to a transfer hose comprising a hybrid reinforcing yarn wherein the reinforcing yarn comprises a co-para-aramid copolymer yarn and a meta-aramid yarn twisted together.
Flexible, low-pressure hose is used in a variety of fluid transfer applications such as in automotive radiator and heater hose and charge air hoses for connecting turbochargers to charge air coolers (“CAC”). By “low-pressure” is meant a hose that meets a maximum working pressure rating in the vicinity of up to about 20 bars, or up to about 350 psi, according to general usage in the field. Flexibility permits the hose to accommodate movement, vibration, installation misalignment, and thermal expansion and contraction. Flexibility is provided by rubber, elastomer, and/or plastic materials used to form the bulk of the hose. Reinforcement is necessary to withstand internal pressures and/or external forces. Textile yarns or fabrics, plastics, and metals are generally used as reinforcements in hose. Reinforcements are applied in one or more layers between an inner tube and an outer cover. Textile yarn reinforcements are applied to the inner tube by braiding, knitting, spiraling, or wrapping in one or more plies or layers. Knit reinforcement provides less strength than braiding, spiraling, or wrapping, as well as a tendency to unravel if a yarn is broken. Also, knitting results in looped yarns of very small bend radius, with high compressive forces on the inside of the bends, which tends to cause kink bands and reduced tensile strength in high-modulus fibers such as para-aramids. Knitting also involves yarns looped around other yarns, creating friction points where the yarns can cut or abrade through each other. Strength loss during the knitting process is significant so that para-aramid knit reinforced hose does not have the initial burst strength one would expect from such reinforcement. Nevertheless, knit reinforcements are often preferred for shaped or curved low-pressure hoses, because knits provide a very flexible type of fabric reinforcement.
The hose performance requirements for dynamic fatigue resistance, temperature resistance, and internal pressure-carrying capability have increased dramatically for some applications. The need to reduce costs is also ever present. For example, modern automotive and truck applications, such as radiator, coolant and CAC hoses, face increased demands. Higher strength and modulus cords, including those formed of polyethylene naphthalate (PEN), poly(p-phenylene-2,6-benzobisoxazole) (PBO), para-aramid, carbon, or liquid crystal polymer (LCP) are known to exhibit higher pressure carrying capability, but are known to exhibit relatively poor dynamic fatigue resistance, particularly after undergoing the knitting process and when tightly bent at friction points as mentioned above in connection with knitting. Typical commercial CAC hose utilizes multiple layers of wrapped fabric textile reinforcement and/or metal reinforcing bands to achieve sufficient burst pressure ratings. Among known textiles in use for CAC hose are multi-ply woven fiberglass fabric, multi-ply NOMEX® or polyester, fabric plies of KEVLAR® or Twaron® para-aramid blended with NOMEX® meta-aramid. Among known textiles in heater hose are nylon, KEVLAR®, and NOMEX® braided reinforcements.
Aramid yarns are widely used for reinforcement in hoses. An example is U.S. Pat. No. 6,742,545 wherein multiple reinforcement layers and protective jackets are combined and a variety of materials recommended, including aramids, in order to construct a high-pressure industrial transfer hose capable of withstanding high external pressures. There is no suggestion as to how to improve the performance of a simpler, aramid-knit-reinforced, low-pressure hose without adding additional layers of reinforcement.
In order to improve the retained strength after use, it is known to use composite cords composed of a core yarn and a sheath of twisted yarns plied around the core. An example is U.S. Pat. No. 4,912,902. The sheath is preferably aramid, which imparts high strength to the cord, but would not be expected to improve the self-cutting problem in a knit reinforcement. Fabrication of the core-sheath construction adds cost and/or complexity to the reinforcement.
In an unrelated field, it is known to combine various high-performance, fire-resistant fibers, such as para- and meta-aramids, to improve knitted fabrics for fire-resistance, breathability, and flexibility in protective apparel or upholstery. An example is U.S. Pat. No. 5,091,243 wherein core-sheath yarns having a core of glass, KEVLAR®, NOMEX®, carbon, polybenzimidazole, metals, etc. or blends thereof, and a sheath of staple cotton, polyester, rayon, wool or the like, are utilized in a fire barrier upholstery fabric. It is not known or suggested to use flame barrier fabrics as encapsulated reinforcement in fluid transfer hoses for improved dynamic performance.
Thus, the prior art fails to disclose a reinforcing yarn with sufficient strength and flexibility to endure a knitting process and to provide a knit-reinforced hose with good impulse fatigue resistance and sufficient burst strength to meet modern automotive requirements for low-pressure, fluid-transfer hose. Neither does the art teach a low-pressure, fluid transfer hose reinforced with a knit reinforcement comprising a hybrid yarn of a blend of co-para-aramid and meta-aramid fibers.